A substitute in the sport of cricket is a replacement player that the umpires allow when a player has been injured or become ill, after the nomination of the players at the start of the game. The rules for substitutes appear in Law 24 of the Laws of Cricket.

A substitute can act for the injured or ill player in the field, although they may not bowl, bat or act as captain, unless otherwise agreed by the captains. A player may bat, bowl and field even if they have had a substitute for part of the game, though they need to wait for a period equal to their time off the field until they bat or bowl again. Substitutes are generally not listed in the official squad list, unless they were in the starting XI for other games in the wider squad, or were the twelfth player, as some forms of cricket have 12 players listed prior to the match, at which point one player is omitted from the match squad. However, this is not required under the laws, and the substitute can be anyone, providing the umpires approve the use of a substitute. This can lead to members of the crowd, on-site media or a team's coaching staff taking the role. One example being a New Zealand journalist who was following their tour of India in 1988, taking the field.

The use of substitutes is known from the 18th century. In the report of a match on Monday, 5 September 1748, the role is termed a "seeker-out"; this was in the sense that George Smith, who was carrying an injury and had been granted a substitute fielder in previous matches, was denied one in this match. The practice of allowing substitutes was varied and inconsistent in the 19th century. In 1870, a match occurred between the MCC and Nottinghamshire. Substitutes were allowed early in this match, but this was condemned by the press, and by the conclusion of the match they were refused.

The first-ever use of a substitute in first-class cricket occurred in The University Match between Oxford and Cambridge in 1891, when Thomas Case replaced Frederic Thesiger in the Oxford XI, after Thesiger had injured himself while fielding on the first morning of the match.

In 2017 the proscription on substitutes being used to wicket-keep was officially lifted. Prior to that only one of the remaining 10 players would be able to do so. Despite the proscription being within the rules there were rare occasions in high level cricket where with the permission of the opposition captain and the umpires, a substitute was allowed to act as wicket-keeper. An example of this was in a Test match in 1986 where Bob Taylor, who had been retired from test cricket for 2 years, performed the duty as an emergency substitute for Bruce French after he was hurt while batting.

A substitute is permitted to take catches as with any other fielder, and on some occasions does. The first occasion in Test cricket was in the Test between England and Australia in 1884, when Australia's captain, Billy Murdoch, took a catch from his teammate Tup Scott while playing as a substitute for England. The highest number of catches by a substitute fielder in a Test match is four, a record shared by Gursharan Singh, Younis Khan, Virender Sehwag, and Jackson Bird. However, substitute fielders' catches do not count towards individual stats.

In recent years, there have been arguments made for cricket to allow substitutes in first-class games, as cricket forbids full substitutes for most injuries or tactical reasons. In 2019, concussion substitutes were allowed in Test matches and other international games for the first time. Arguments in favour of general substitutes have been made from a perspective of improving the game, coping with increasing injury rates due to the modern schedule, to follow the lead of other sports in trying to manage concussion more responsibly and to provide greater opportunities for players to gain experience in first-class cricket. However, there is an equally strong viewpoint that the nature of the first-class contest may be diminished with a summary suggesting that although medical experts would recommend introduction of substitutes the majority of players are still not in favour.

When a player leaves the ground due to injury and is replaced by a substitute fielder, they are generally not permitted to return and immediately resume bowling (or batting if their team's innings commences while they are off the field). The injured player is required to spend a period back on the field at least equal to the time that they were absent before resuming bowling, or batting should a change of innings occur. Variations of the time periods required and the circumstances of the players return to the field apply in different forms of the game.